Demand for a low-cost snapshot multi spectral imager has been increasing for various applications, which include accurate color reproduction, machine/robot vision, plant and vegetation research, food processing, counterfeit detection, early stage diagnosis of cancer, medical in-vivo imaging, and defense applications (point/stand-off optical spectral detection systems for remote sensing). Especially, accurate color reproduction is highly desired for a growing number of smart displays equipped with color camera modules and color displays.
A typical multispectral imager essentially consists of either rotating filter wheels, mechanically diced thin-film dichroic filters mounted in front of an image sensor, or multiple cameras with bulk dichroic filters. Even for those touted as commercial systems, there is no real volume production pathway with significant price or reduced complexity enhancements for as few as tens or hundreds of units.
There is a low cost color filter array used for typical CCD or CMOS image sensor which is a negative type photosensitive material that can be patterned with UV light. It consists of pigments to define the spectrum of the color filter, a dispersant polymer for pigment dispersion, an initiator to generate the radical for the polymerization reaction, a monomer to be polymerized and an alkaline soluble polymer to control the development property. The photo-polymerization starts with the radicals generated when the initiator is exposed to UV light. When the radical gets in contact with the monomers, the polymerization starts and forms the high molecular weight polymer insoluble for the developer. The un-exposure area is not polymerized and is removed during the development process. As a result, the pattern profile is formed. This type of color filter array requires coating, pre-bake, exposure, development, rinse and post-bake multiple times to make a mosaic pattern. Although this type of filter can be made at low cost for one filter, the cost increases as number of filter type of an array increases. Also the bandwidth is broad and the band selectivity is limited to a visual range.
Recently, it has been found that certain nanostructures work as an optical filter, and has a strong advantage compared to prior technologies. Multiple, or almost infinite number of, optical filters can be made on a single layer, at no additional cost. The spectral shapes of these plasmonic nano-optical filters can be controlled. It can be narrowed but usually at an expense of the transmission power. Also usually unwanted second peaks or third peaks are generated at relatively high transmission power. For the above reasons, low cost and clean spectral filters are not readily available for multi-spectral imager application so far.